1. Field of the Invention
The invention relates to a diagnostic system and diagnostic method for an internal combustion engine. Specifically, the invention relates to a technique for carrying out a diagnosis of a difference in air-fuel ratio among a plurality of cylinders in an internal combustion engine in which an injector that injects fuel inside a corresponding one of cylinders and an injector that injects fuel outside a corresponding one of the cylinders are provided in correspondence with each of the cylinders.
2. Description of Related Art
There is known an internal combustion engine in which an injector that injects fuel is provided in correspondence with each of a plurality of cylinders. In such an internal combustion engine, for example, if the injection amount of the injector becomes insufficient in only part of the cylinders, the air-fuel ratio of each of the part of the cylinders may deviate from the air-fuel ratio of the other one of the cylinders. When there is a difference in air-fuel ratio among the cylinders, rotation fluctuations of an output shaft of the internal combustion engine increase, so large vibrations may be generated. Thus, in order to be able to notify a user of an abnormality and to prompt the user for necessary repair, it is desired to detect the air-fuel ratio is not uniform.
Japanese Patent Application Publication No. 2008-14198 (JP 2008-14198 A) describes a technique for detecting an abnormality in a fuel injection system in an engine including a port injector that injects fuel into an intake port and a direct injector that directly injects fuel into a combustion chamber. JP 2008-14198 A describes the following abnormality diagnosis at paragraphs 21 to 25. That is, in the case where fuel is being injected in a distributed manner from the port injector and the direct injector, when a fluctuation amount of output torque is larger than a predetermined value, it is determined whether the direct injector is abnormal by injecting fuel with the use of only the direct injector. When it is determined that the direct injector is abnormal, fuel injection is switched to fuel injection with the use of only the port injector, and it is determined whether the port injector is abnormal on the basis of the fluctuation amount of output torque.
However, in JP 2008-14198 A, when the direct injector is normal, abnormality determination is not made in a state where fuel injection is carried out with the use of only the port injector. Thus, the accuracy of determination as to whether the port injector is abnormal may be low.